WM8739 / WM8739L

Advanced Information

AI Rev 2.2 September 2001

15

For applications where a component other than the WM8739 will generate the reference clock, the

external system can be applied directly through the XTI/MCLK input pin with no software

configuration necessary. Note that in this situation, the oscillator circuit of the WM8739 can be safely

powered down to conserve power (see POWER DOWN section)

CRYSTAL OSCILLATOR

The WM8739 includes a crystal oscillator circuit that allows the audio system’s reference clock to be

generated on the device. The crystal oscillator is a low radiation type, designed for low EMC. A

typical application circuit is shown in Figure 12.

XTI/MCLK

XTO

DGND

DGND

Cp

Cp

Figure 12 Crystal Oscillator Application Circuit

For crystals with a 30pF fundamental load capacitance, a value of 5pF for Cp is recommended.

The WM8739 crystal oscillator provides an extremely low jitter clock source. Low jitter clocks are a

requirement for a high quality audio ADC, regardless of the converter architecture. The WM8739

architecture is less susceptible than most converter techniques but still requires clocks with less than

approximately 1ns of jitter to maintain performance. In applications where there is more than one

source for the master clock, it is recommended that the clock is generated by the WM8739 to

minimise such problems.

CORE CLOCK

The WM8739 DSP core can be clocked either by MCLK or MCLK divided by 2. This is controlled by

software as shown in Table 4 below.

REGISTER

ADDRESS

BIT

LABEL

DEFAULT

DESCRIPTION

0001000

Sampling

Control

6

CLKIDIV2

0

Core Clock divider select

1 = Core Clock is MCLK divided by 2

0 = Core Clock is MCLK

Table 4 Software Control of Core Clock

Having a programmable MCLK divider allows the device to be used in applications where higher

frequency master Clocks are available. For example the device can support 512fs master clocks

whilst fundamentally operating in a 256fs mode.

DIGITAL AUDIO INTERFACES

WM8739 may be operated in either one of the 4 offered audio interface modes. These are:

•

Right justified

•

Left justified

•

I

•

DSP mode

All four of these modes are MSB first.